The present invention relates to an automatic cooking control system for a microwave oven with a turntable which can automatically cook food by utilizing a temperature sensor that detects the temperature of air flowing out of a heating chamber, and more particularly, to an automatic cooking control system which is able to automatically cook food by establishing a heating time of for the food to be cooked even though an outflow air temperature detected at the temperature sensor oscillates due to the rotation of the turntable.
In general, a microwave oven which cooks food automatically is constructed, as shown in FIG. 1, with a microcomputer 1 which controls the whole operation of a microwave oven, a power source 2 which supplies electric power under the control of the microcomputer 1, a magnetron 3 which generates microwave energy upon actuation by electric power from the power source 2, a heating chamber 4 which heats the food positioned on a turntable 4A with the microwave energy generated for the magnetron 3, a fan 5 which blows air through an air inlet 4B into the heating chamber 4, a temperature sensor 6 which detects the temperature of air flowing out through an air outlet 4C of the heating chamber 4, and an analog/digital converter 7 which converts the signal of outflow air temperature detected by the temperature sensor 6 into a digital signal and inputs the converted signal into the microcomputer 1.
With the conventional microwave oven constructed as described above, when a food to be cooked is put onto a turntable 4A of a heating chamber 4 and an automatic cooking is started by pressing a cooking start button, a microcomputer 1 begins to execute an initial operation for a predetermined time t.sub.1 as shown in FIGS. 2 and 3. A fan 5 is actuated for about sixteen seconds to blow air through an air inlet 4B into the heating chamber 4 so that the air temperature of the heating chamber 4 can be made uniformed. The temperature of the air flowing out of the air outlet 4C is detected by a temperature sensor 6. The detected temperature signal is then converted into a digital signal by the analog/digital converter 7.
When a predetermined time t.sub.1 has elapsed, the microcomputer 1 receives and stores the signal of the present temperature T.sub.1 which is outputted from the analog/digital converter 7. The microcomputer 1 controls the actuation of the power source 2, The food positioned on the turntable 4A of the heating chamber 4 is heated by microwave energy generated by the magnetron 3. Since the temperature of air flowing out of the heating chamber 4 through the air outlet 4C is gradually raised according to the heating of food, the temperature detection signal, which is detected by the temperature sensor 6 and inputted to the microcomputer 1 through the analog/digital converter 7, is also gradually raised.
The temperature increment is raised as much as a predetermined value .DELTA.T. The temperature detected at the temperature sensor 6 is raised as much as a predetermined temperature T.sub.2 so that when the temperature increment becomes a predetermined value .DELTA.T, microcomputer 1 finishes a first stage heating operation and starts to execute a second stage heating.
In summary, the conventional automatic cooking control is executed utilizing a method having the steps of: storing a time t.sub.2 of a first stage heating; calculating a second stage heating time t.sub.3 by multiplying the first stage heating time t.sub.2 by a predetermined value .alpha. established in accordance with the kind of food to be cooked; heating the food by continuously actuating the magnetron 3 for the second stage heating time t.sub.3 ; and completing the cooking of the food by stopping the actuation of magnetron 3 and fan 5 when the second stage heating time t.sub.3 has elapsed.
In such an automatic cooking control method, since the geometrical center of the turntable 4A and the temperature-responsive center of the food to be cooked in the course of the rotation of the turntable 4A are not in precise accord with each other, the temperature characteristic of outflow air detected by the temperature sensor 6 oscillates.
FIG. 4 is a graph showing a temperature response characteristic of the outflow air when cooking egg custard comprising two eggs with two cups of milk. The temperature response characteristic of outflow air oscillates causing the first stage heating time to become short and the second stage heating time to become short, thereby causing the automatic cooking process; not to be correctly performed.
The outflow air temperature oscillates as shown in FIG. 5. The first stage heating process is finished at the time t.sub.a, but not the desired time causing the first stage heating time to be shortened as much as a predetermined time .DELTA.t.sub.1. Thus the second stage heating time also becomes short, thereby making it impossible to execute correctly the automatic cooking of food.